A conventional hydraulic drive device for construction machinery, such as a hydraulic excavator, disclosed in, for example, Japanese Patent Laid-Open Publication No. 2004-36681 (Patent document 1) includes: a variable capacity type first hydraulic pump driven by a drive source such as an engine; a first hydraulic actuator group driven by the pressure oil discharged by the first hydraulic pump; a first operating valve group interposed between the first hydraulic pump and the first hydraulic actuator groups; a variable capacity type second hydraulic pump driven by the drive source mentioned above; a second hydraulic actuator group driven by the pressure oil discharged by the second hydraulic pump; and a second main operating valve group interposed between the second hydraulic pump and the second hydraulic actuator group. This hydraulic drive device connects a pressure oil supply line of the first hydraulic pump and a pressure oil supply line of the second hydraulic pump via a first merging/diverging valve, and controls switching of the first merging/diverging valve, thereby switching each pressure oil supply line to a merging position or diverging position.
In addition, according to Patent document 1, in order to relieve shock that may occur during switching to a merging position or diverging position, a bypass oil path is disposed so as to connect, via a pressure compensation valve with a checking function, an oil path between the pressure compensation valve on the one hydraulic pump side and the actuator and an oil path between the other variable capacity type hydraulic pump and the main merging/diverging valve. By disposing the bypass oil path in such a manner, pressure oil can be kept flowing in a supplied-side hydraulic circuit from a supplying-side hydraulic circuit via the bypass oil path when the merging/diverging valve is switched to the diverging position from the merging position. This makes it possible to avoid a change in flow rate during the switching to the merging or diverging position, or any resultant shock. Accordingly, this prevents noises emitted when shock occurs, or operation degradation resulting from a change in flow rate or pressure.
Patent document 1: Japanese Patent Laid-Open Publication No.